One type of engine performance problem found in internal combustion engines is poor cylinder contribution. In a properly functioning engine, each cylinder contributes equally to the total output power for balanced operation. One problem with conventional diagnostic instruments that measure cylinder contribution is that they often require complicated or time-consuming connections to the engine. Further, conventional diagnostic instruments may display results that are difficult to interpret or provide only general information. That is, a technician may invest time setting up a diagnostic instrument yet not receive specific enough information about a problem or performance issue to perform a repair efficiently.
Another conventional technique for measuring engine cylinder contribution derives information about cylinder contributions from the alternator output signal. This technique typically involves a simple electrical connection at the alternator, battery, or electrical accessory receptacle (e.g., a cigarette lighter receptacle). In a typical engine system, a crank pulley on the engine drives an associated alternator pulley using a belt. The alternator pulley is coupled to the alternator rotor, which rotates to generate electrical energy. A typical alternator internally generates a multiphase (e.g., 3 phase) alternating current (AC) signal. This multiphase signal is rectified by a number of diodes, such that the alternator outputs a mostly direct current (DC) signal. The alternator output signal also includes, however, a small AC component known as the ripple voltage or diode ripple signal.
Engine speed varies slightly as each cylinder fires. More specifically, the engine accelerates immediately after a cylinder firing and then decelerates until another cylinder firing occurs. Cylinder contribution can be assessed therefore by analyzing the instantaneous variations in engine speed. The diode ripple signal is proportional to the instantaneous speed of the engine because the alternator is coupled to the engine's crankshaft. That is, variations in the engine speed modulate the diode ripple signal in a mathematically meaningful manner.
In some prior approaches, the diode ripple signal is demodulated using a frequency modulation (FM) receiver or analog phase locked loop. One problem with these approaches, however, is the accuracy or precision of the resulting demodulated signal. The diode ripple signal is typically corrupted by electrical noise or other signals that emanate from various sources. For example, the alternator itself may produce undesirable transients and amplitude modulations in the diode ripple signal. These noise signals can make it difficult to demodulate the diode ripple signal accurately or precisely. Given that an engine performance problem is likely attributable to the malfunction of a single engine cylinder, a technician does not want to receive inaccurate information and, therefore, consume diagnostic or repair time on a properly functioning cylinder.
What is needed is a system and method for demodulating the diode ripple signal that is robust in the presence of amplitude variations and impulse impairments. What is further needed is a system and method that filters the alternator output signal to reduce interference.